1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly to a multi-layer super resolution optical disc, which has enhanced signal quality and enhanced recording density.
2. Description of the Related Art
Generally, currently available optical discs have a storage capacity of about 4.7 GB (gigabytes) in the DVD (Digital Versatile Disc) standard using red laser or of about 25 GB in the BD (Blue-ray Disc) standard using blue laser, which has been commercialized recently. However, in order to record and reproduce a huge amount of information for applications, such as high definition TV or E-medicine, there is a need to provide an information storage technology with a higher density.
For instance, in order to record a data stream of high definition digital video, recently available on the market, there is a need to provide a storage medium, which has a storage capacity of 20 GB or more and is able to record the data at a speed of 25 Mbps or more.
Moreover, it is anticipated that technology capable of recording 100 GB or more of information/CD-size media will be required after 2005 and technology capable of recording terabytes of data on CD-size media will be required after 2010. For this purpose, it is necessary to record the information at a high density and a high speed.
In such various multimedia environments, research and development of various types of multi-functional information storage technology has been conducted.
Among the information storage technologies, optical recording technology is most spotlighted and widely used due to its merits. Specifically, an optical recording medium is capable of being easily detached from its drives, and storing a large amount of information data. As a necessary function in the multimedia environment, random access of data on the optical recording medium is allowed in the optical recording technology. Further, high data integrity and low manufacturing costs are provided.
As a method for increasing the recording density of the optical recording medium, a method of decreasing the size of the laser beam incident on the recording medium is used. As developed at present, the reduction of the size of the laser beam incident on the recording medium is accomplished by decreasing the wavelength of the laser or increasing a numerical aperture of an objective lens. The size of bits recorded on the medium is in proportion to the wavelength of the laser, and is in inverse proportion to a numerical aperture (NA) of an objective lens.
However, currently, methods for increasing recording density by using a short wavelength, such as a blue laser (405 nm), together with a high numerical aperture (NA=0.85), have substantially arrived at the theoretical limits of optics, so there is a need to develop a new technology for realizing a higher storage capacity.
Accordingly, as a plan for developing an optical memory, which is compatible with the existing CD or DVD and is also capable of storing information with a density several hundreds times higher than the storage capacity of the existing CD, that is, 650 MB, research into an optical disc using a super resolution phenomenon, that is a super resolution optical disc, has been conducted.
It is expected that the super resolution technology can remarkably reduce the size of a recording mark while using a conventional laser optical pickup system, thereby increasing the recording density. The super resolution technology uses a near-field effect by a focused laser beam.
Among the super resolution technologies, according to a WORM (Write Once Read Many)-type super resolution technology, recording marks are not formed on a crystal/non-crystal reversible phase transformation-type recording layer available for the DVD. Instead, an oxide thin film, such as AgOx, PtOx and the like, of a mask layer in a recordable optical disc is decomposed by the laser beam, and cavities formed from the decomposition and nanosized metal particles precipitated in the mask layer form near-field while acting as scattering cores such that the cavities and particles act as a recording layer.
Here, dielectric thin films consisting of a ZnS—SiO2 based component are formed at upper and lower portions of the oxide thin film of the mask layer, respectively, in consideration of optical, thermal, and mechanical properties of the oxide thin film. Further, as for a substrate, polycarbonate (PC) is mainly used, since PC is light, has good injection properties, and can increase carrier-to-noise ratio (CNR) due to a low birefringence when the laser is incident thereon.
FIG. 1 is a view illustrating the structure of a conventional super resolution optical disc. By irradiating the laser beam on the super resolution optical disc, the thermally decomposed marks are locally recorded in the thin film laminated on a groove or a land, and by irradiating a low power laser thereon, a signal caused by difference of reflection rate between the mark and the background (space), which is not formed with the marks, is detected.
At this time, a jitter value becomes an important standard for demonstrating a reproduction quality. The jitter value is an amount represented by a statistical variation of the values of time differences between a clock (PLL clock) generated from a reproduction signal and a border of each recording mark determined from the reproduction signal. Specifically, the jitter value is a value represented by a percentage of the standard deviation of the values of time differences between a leading edge of the mark and the PLL clock and between a trailing edge of the mark and the PLL clock, divided by 1 T.
Although there are various causes influencing the jitter value and various aspects influencing the jitter value are not simple, especially, Inter-Symbol Interference (ISI) caused by the shape of the marks formed on the recording medium and by relation of marks and spaces between the marks plays a very important role in influencing the jitter value. That is, in a process for analyzing the signal read by a pickup unit, the relation between the shape of the mark recorded on the recording medium and the space between the marks determines the quality of the reproduction signal. Accordingly, it is very important to provide a technology which can record the marks on the recording medium with a desired shape in order to reduce the jitter value by improving the quality of the reproduction signal.
According to research into the conventional super resolution optical disc using a single mask layer, as shown in FIG. 2, the mask layer, which is several nanometers thick, [for example, PtOx (4 nm)], is expanded to have a thickness of ten times or more through the recording process, and continues to expand through the reproduction process after recording. An expanded portion is a Ge—Sb—Te or Ag—In—Sb—Te thin film layer, which is used as a recording layer in a typical phase transformation type optical disc. The expansion is attributed to the fact that the mask layer is decomposed by the focused laser beam and a phase transformation material having a low melting point is partially melted or decreased in viscosity, thereby causing the remaining mask layer to easily expand toward the decomposed portion thereof.
Further, as shown in FIG. 3, in case of a super resolution optical disc 101, which has a symmetrical structure of a dielectric layer (for example, ZnS—SiO2) and a recording layer (for example, Ag—In—Sb—Te) at upper and lower portions of the mask layer (for example, PtOx), each mark in the mask layer is expanded into an oval-shape.
Here, since the laser beam has a temperature variation corresponding to a Gaussian distribution at the cross section thereof, there is a considerable difference of volume expansion between the center of the recording marks and an outer periphery thereof, and the degree of volume expansion is gradually decreased toward the outer periphery of the recording marks.
Due to such a non-uniform expansion of the mask layer, the recording marks are not uniform, and the reflection rate differs between the center of the marks and the outer periphery thereof. Moreover, the border between the outer periphery of the marks and the space between the marks is poorly defined. Accordingly, there are problems in that the quality of the reproduction signal of the optical disc is lowered and in that integrity of reproduction performance cannot be secured.
Moreover, although there have been continuous efforts to increase the storage capacity of the super resolution optical disc, an effective solution for this problem has not been discovered yet.